CMake: add our own FindCygwin module

The one from official CMake (3.3.2) does not look for the default 64bits installation path. Let's add it.
Also add a WIRESHARK_CYGWIN_INSTALL_PATH environment variable allowing to force it.
For reference, registry based detection fails to detect a 64bits installation because it gets redirected to the Wow6432 node.

Change-Id: If3172494e3ab232e094389b493e6b67023662ae5
Reviewed-on: https://code.wireshark.org/review/11769
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
Reviewed-by: Graham Bloice <graham.bloice@trihedral.com>

cmake/modules/FindCygwin.cmake

@@ -0,0 +1,65 @@
+#.rst:
+# FindCygwin
+# ----------
+#
+# this module looks for Cygwin
+
+# This code was copied from
+# http://cmake.org/gitweb?p=cmake.git;a=blob_plain;f=Modules/FindCygwin.cmake;hb=HEAD
+# and modified so as to check C:\Cygwin64 and the WIRESHARK_CYGWIN_INSTALL_PATH
+# environment variable
+
+#=============================================================================
+#CMake - Cross Platform Makefile Generator
+#Copyright 2000-2015 Kitware, Inc.
+#Copyright 2000-2011 Insight Software Consortium
+#All rights reserved.
+#
+#Redistribution and use in source and binary forms, with or without
+#modification, are permitted provided that the following conditions
+#are met:
+#
+#* Redistributions of source code must retain the above copyright
+#  notice, this list of conditions and the following disclaimer.
+#
+#* Redistributions in binary form must reproduce the above copyright
+#  notice, this list of conditions and the following disclaimer in the
+#  documentation and/or other materials provided with the distribution.
+#
+#* Neither the names of Kitware, Inc., the Insight Software Consortium,
+#  nor the names of their contributors may be used to endorse or promote
+#  products derived from this software without specific prior written
+#  permission.
+#
+#THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+#"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+#LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+#A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+#HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+#SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+#LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+#DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+#THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+#(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+#OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#=============================================================================
+
+if (WIN32)
+  find_path(CYGWIN_INSTALL_PATH
+    cygwin.bat
+    PATH ENV WIRESHARK_CYGWIN_INSTALL_PATH
+    "C:/Cygwin"
+    "C:/Cygwin64"
+    "[HKEY_LOCAL_MACHINE\\SOFTWARE\\Cygwin\\setup;rootdir]"
+    "[HKEY_LOCAL_MACHINE\\SOFTWARE\\Cygnus Solutions\\Cygwin\\mounts v2\\/;native]"
+  )
+
+  if(${CYGWIN_INSTALL_PATH} STREQUAL "CYGWIN_INSTALL_PATH-NOTFOUND")
+    message(FATAL_ERROR "Cygwin installation path was not detected. You can set it with WIRESHARK_CYGWIN_INSTALL_PATH environment variable.")
+  else()
+    mark_as_advanced(
+      CYGWIN_INSTALL_PATH
+    )
+  endif()
+
+endif ()

docbook/wsdg_src/WSDG_chapter_quick_setup.asciidoc

@@ -387,6 +387,13 @@ Set the following environment variables, using paths and values suitable for you
 
 > set WIRESHARK_VERSION_EXTRA=-YourExtraVersionInfo
 ----
+If your Cygwin installation path is not automatically detected by CMake, you can explicitly specify it with the following
+environment variable:
+
+----
+> set WIRESHARK_CYGWIN_INSTALL_PATH=c:\cygwin or whatever other path that is applicable to your setup
+----
+
 If you are using a version of Visual Studio earlier than VS2012 then you must set an additional env var,
 e.g. for VS2010 set the following:
 ----